Process for pulverising polyurethane, polyurea and/or polyurethane/polyurea materials in a roller mill

ABSTRACT

When pulverising polyurethane, polyurea and/or polyurethane/polyurea materials in a roller mill with at least two rollers (2,3) rotating at different circumferential speeds, a very economical method of operation is achieved by returning the coarse fraction falling freely out of the roll slit (4) to the roll slit (4) and stripping the fine material adhering to the roller (3) with the higher circumferential speed and discharging it separately from the coarse fraction.

The invention relates to a process for pulverising polyurethane,polyurea and/or polyurethane/polyurea materials in a roller mill whoseat least two rollers rotate at different circumferential speeds, whereinthe material to be milled is fed to the roll slit formed by the rollersand after its passage is separated into a coarse fraction and a finefraction, the coarse fraction being returned to the roll slit and thefine fraction being passed on for further processing.

In recent years, the recycling of plastics has obtained a very highstatus. Possibilities have even been sought for the material recyclingof cross-linked polyurethanes, such as are used, for example, for highlystressed motor vehicle bumpers. For recycling, it is generally necessaryto comminute, and in particular to mill, these scrap materials in orderto be able to re-use them in powder form, in particular as fillers.Particularly fine pulverisation is required if they are to be added asfillers to formulations for polyurethane reactive systems. The use ofroller mills is known (DE 42 07 972-Al). Milling leads to a mixture of alarge quantity of very coarse and a small quantity of already fineparticles. The coarse particles are screened out and returned to the gapbetween the rollers. This screening process is very uneconomical becausethe screen is loaded with a large quantity of very large particles andthus comparatively small quantitites of fine material are obtained.

The object of the present invention is to improve the previously knownprocess in order to make it more economical.

This object is achieved by collecting and returning the coarse particleswhich fall freely downstream of the roll slit, while the fine fractionwhich remains as a sheet adhering to the roller with the largercircumferential speed is discharged separately from the coarse fraction.

According to the known process, both rollers were cleaned with a scraperso that all the milled material was fed to the screen. In contrast,according to the invention, the sheet consisting of the fine fraction,which forms on the roller which rotates more rapidly during thepulverisation of the materials mentioned above, can be utilised so thatthis sheet is no longer passed to the freely falling coarse fractions,but is discharged separately.

Obviously the sheet also contains some coarse fractions, but theparticle size spectrum is very favourably shifted to the finer end.

For many possibilities for recycling, the fine fraction can therefore bedirectly re-used. However, if a particularly fine powder is required,then the sheet consisting of the fine fraction is preferably classifiedagain into coarse and fine particles. Known screening methods can beused for this purpose. When screening in this process step, the finefraction is much larger and may even be larger than the coarse fraction.If there is a possibility of direct re-use for the now relatively smallcoarse particles then these are discharged, but on the other hand theymay be returned to the roll slit for re-milling.

It is understood that the sheet may also be stripped off and fed toanother roll slit.

One particularly advantageous variant comprises feeding the sheetadhering to the roller, by means of this roller, directly to the rollslit formed between this roller and a third roller.

However, since this third roller has to be arranged in such a way thatit rotates upwards, the coarse fraction here cannot--as in the firstroll slit--be separated by free falling. Thus, if the coarse particlesare required to be isolated once again after passage through the secondroll slit, screening must be carried out.

Preferably, the coarse fraction to be recirculated is cooled.

There is considerable friction during the comminuting process and heatis therefore produced. Since some of the particles in the coarsefraction pass through the roll slit several times, the milled materialwould become hot, and thus damaged, if no cooling were provided.

The starting materials for performing the process are preferably apolyurethane flexible foam, a polyurethane structural foam or a RIMmaterial made of polyurethane, polyurethane/polyurea or of polyurea.These materials are distinguished by having a particularly pronouncedtendency to form a sheet during milling. Powders from these materialsare however particularly suitable for recycling.

The powders produced by the new process can be fed to conventionalthermoplastic processing and relatively easily processed and blendedtogether with the thermoplastics. Extremely finely powdered polyurethanecan be used advantageously in chemolytic processes such as e.g.glycolysis, aminolysis and hydrolysis. The powdered form is alsoadvantageous for extrusion. This is especially true when powder fromlacquered RIM structural parts is used as starting material.Polyurethanes can only be introduced in the form of very fine powders asfillers into polyurethane reaction mixtures to be re-processed. Thepowder produced according to the invention can also advantageously beused for thermal processing techniques.

The preparation of the polyurethane, polyurea and/orpolyurethane/polyurea materials from which the powder is subsequentlyobtained for recycling, takes place by methods known per se frompolyurethane chemistry. They are non-melting, and therefore thermoset,materials. From which chemical formulations these materials are obtainedand how they are processed to form articles (bumpers, instrument panels,side-aprons, spoilers, etc. for the motor vehicle industry, furniture,housings etc.) is described, inter alia, in Kunststoff-Handbuch,Polyurethane, 2nd ed., Carl Hanser Verlag, Munich/Vienna (1983).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an embodiment of the process wherein theprecomminuted scrap material is fed through a roll slit and separatedinto a coarse fraction and a sheet of fine particles.

FIG. 2 is a schematic view of another embodiment of the process whereina second set of rollers is located thereunder the feed hopper situatedunder the first set of rollers.

FIG. 3 is a schematic view of another embodiment of the process whereinthe front view of a mill is shown without the first set of rollers.

FIG. 4 is a schematic side view of mill shown in FIG. 3.

FIG. 5 is a schematic view of another embodiment of the process whereina third roller in combination with the roller having a highercircumferential speed forms a second roll slit.

FIG. 6 is a schematic view of another embodiment of the process whereinthe coarse fraction is returned to the feed hopper, via a cellular wheelsluice by means of compressed air and also via a cyclone.

FIG. 7 is a schematic view of another embodiment of the process whereinthe roller mills consists of three rollers.

The drawings show a purely schematic representation of the process inseveral versions using a diagram of the relevant milling units and theprocess is explained in more detail in the following:

When performing the process in accordance with the diagram shown in FIG.1, scrap material which has been precomminuted to a suitable sizereaches a roll Slit 4 formed between two rollers 2, 3 via a feedhopper 1. The rollers 2, 3 have the same diameter; roller 3 does howeverrotate at a higher circumferential speed. A sheet 5 of finer particlesis formed on roller 3, while the coarse fraction falls freely into acollecting hopper 6 and is returned to feed hopper 1 by means of aconveyer 7, for example by means of a drag chain. The sheet 5 isdetached from roller 3 by a scraper 8 and is delivered to screeningdevice 10 via chute 9. Roller 2 is also cleaned of residues by scraper11, and these are also delivered to screening device 10 via chute 12.The screened coarse fraction is returned to feed hopper 1 via conveyer13, while the fine fraction is removed via outlet 14 and is fed to atemporary storage facility or passed on directly for further processing.If the particles of the fine fraction obtained from sheet 5 aresufficiently small, screening can even be omitted and they can bedirectly re-processed. In this case screening device 10 and conveyer 13are superfluous.

When performing the process in accordance with the diagram shown in FIG.2, scrap material which has been precomminuted to a suitable sizereaches a roll slit 24 formed between two rollers 22, 23, via a feedhopper 21. The rollers 22, 23 have the same diameter; roller 23 doeshowever rotate more rapidly. A sheet 25 of fine particles is formedthereon, while the coarse particles fall freely into collecting hopper26 arranged thereunder, and are returned to feed hopper 21 via conveyer27. Sheet 25 is detached from roller 23 with a scraper 28 and reaches asecond roll slit 32 formed by two rollers 30, 31, of which roller 31rotates more rapidly, via chute 29. A scraper 33 is also arranged onroller 22, and this removes residues which are also fed to roll slit 32via chute 34. After passage through roll slit 32, the milled materialmay be passed on for further processing.

The milled material may of course also be separated into a coarsefraction and a fine fraction at the second pair of rollers 30, 31--inprecisely the same way as after-the first pair of rollers 22, 23.Scrapers 35, 36 remove residues or a sheet 37 from rollers 30, 31respectively. The milled material is collected in a hopper 38 andwithdrawn through outlet 39. In this way it is possible to return thecoarse fraction to roll slit 32 or to roll slit 24, provided the mill isequipped with the conveyers required for this purpose.

To perform a third variant of the process, FIG. 3 shows the front viewof a mill and FIG. 4 shows the side view. The difference from the millshown in FIG. 2 is essentially that instead of a second pair of rollersbelow the feed hopper 41, only one pair of rollers 42, 43 is used, whichis however longer. The material to be milled which has beenprecomminuted to a suitable size passes through roll slit 44. A sheet 45of fine material forms on the more rapidly rotating roller 43, while thecoarse fraction falls freely into a feed hopper 46 and is returned tofeed hopper 41 via conveyer 47. Sheet 45 is detached from the morerapidly rotating roller 43 by means of scraper 48 and is delivered to asecond area 51 (FIG. 4) located outside the range of feed hopper 41 viachute 49 and conveyer 50. The residues are also removed from roller 42by means of scraper 52 and they also reach conveyer 50 via chute 53.After passing through roll slit 44 in area 51, the coarse fraction fallsinto chute 54. The residue adhering to roller 42 in area 51 and sheet 55located on roller 43 are removed by scrapers 56, 57 and are deliveredtogether to an outlet 58 via chute 54.

When performing the process in accordance with the diagram shown in FIG.5, scrap material which has been precomminuted to a suitable sizereaches roll slit 64 formed by two rollers 62, 63, via feed hopper 61.Rollers 62, 63 have the same diameter; roller 63 does however rotatewith a higher circumferential speed. A sheet 65 of fine particles isformed on roller 63, while the coarse fraction falls freely intocollecting hopper 66 located thereunder and is returned to feed hopper61 by means of a conveyer 67, for example a drag chain. Roller 63 formsa second roll slit 73 with a more slowly rotating roller 72, throughwhich sheet 65 is passed. A scraper 74 strips the residues from roller72. Scraper 68 strips sheet 65 from roller 63 and this reaches screeningdevice 70 via chute 69. The coarse fraction removed by screening isdelivered to feed hopper 61 via conveyer 75; the fine fraction iswithdrawn via outlet 76. Roller 62 is cleaned of residues by scraper 71and these are delivered to conveyer 67 with the coarse fraction. Ifroller 72 rotates more rapidly than roller 63 then at least some ofsheet 65 crosses over onto roller 72 when passing through roll slit 73.

When performing the process in accordance with the diagram shown in FIG.6, scrap material which has been precomminuted to a suitable sizereaches roll slit 84 formed by two rollers 82 and 83, via feed hopper81. The rollers 82 and 83 have the same diameter; the roller 83 doeshowever rotate at a higher circumferential speed. A sheet 85 of finerparticles is formed on roller 83, whereas the coarse fraction fallsfreely into a collecting hopper 86 and is returned, via a cellular wheelsluice 97 by means of compressed air issuing from nozzle 98, through apipe 87 to feed hopper 81 via cyclone 100. The sheet 85 is detached fromthe roller 83 by a scraper 88 and passes to a screening device 90 via achute 89. The roller 82 is also freed of residues by means of a scraper91, which residues are also delivered to screening device 90 through achute 92. The screened coarse fraction is returned, via a cellular wheelsluice 95 by means of compressed air issuing from nozzle 96, through apipe 93 to feed hopper 81 via cyclone 99, whereas the fine fraction isdischarged via outlet 94 and delivered to an intermediate storagefacility or passed on directly for further processing. If the particlesof the fine fraction obtained from the sheet 85 are sufficiently smallscreening can be dispensed with and the particles can be directlyprocessed further. In this case the screening device 90 and components95, 96, 93 and 99 of the conveying device can be dispensed with. If thematerial is conveyed via cooled compressed air the material can beadditionally cooled in addition to the cooling of the rollers.

A further preferred embodiment of the invention is depicted in FIG. 7.The roller mill consists of three adjacent rollers 92, 93 and 94. Thetwo outer rollers 92 and 94 rotate more rapidly than the roller 93 inthe middle. The three rollers form two roll slits 95 and 96. Thematerial to be pulverised falls from the feed hopper 91 first of allinto the first roll slit 95, a first sheet 97 of finely divided materialbeing formed on roller 94. The coarse fractions fall onto the conveyorbelt 98 and are delivered to the second roll slit 96, a second rollersheet 99 being formed on roller 92. Any remaining coarse fractions arereturned to the first slit 95 by means of roller 93.

The conveyor belt 98 rests at least against roller 92, so that no coarsematerial issues between roller 92 and conveyor belt 98. The velocity ofthe conveyor belt 98 corresponds to the circumferential speed of theslow roller 93.

Sheets 97 and 99 are removed from the rollers by means of wipers 100 and101 and are passed onto the screen 102, where finely divided powder isseparated from oversized particles. The oversized particles are returnedto roll slit 95 through channel 103.

EXAMPLE

The preparation of a powder from a foamed polyisocyanate-polyadditionproduct:

a) Preparation of component A

60 parts by wt. of a trifunctional polyether, molecular weight 6000,prepared by the addition of a total of 87% of PO and 13% of EO to TMP,

40 parts by wt. of a filled polyether, molecular weight 6000, preparedby the block-wise addition of 68.75% of PO, 14.58% of EO and 16.66% of aTDI/hydrazine polyaddition product to TMP,

3.8 parts by wt. water,

0.05 parts by wt. catalyst 1,

0.25 parts by wt. catalyst 2,

0.45 parts by wt. catalyst 3,

0.4 parts by wt. diethanolamine cross-linking agent

1.0 parts by wt. of a stabiliser based on a polysiloxane/polyether blockcopolymer

are mixed together and homogenised by stirring.

b) Preparation of component B

TDI 80/20 and PMDI are homogeneously mixed in a ratio of 70:30.

c) Preparation of the polyisocyanate/polyaddition product

105.95 parts by wt. of component A are mixed with 48.5 parts by wt. ofcomponent B and homogenised for 5 seconds with a laboratory stirrer(diameter of the stirring blade ca. 5 cm) at 4200 min⁻¹. The reactionmixture is then immediately placed in a mould treated with a wax-basedrelease agent and pre-heated to 55° C. and the mould is closed. After 10minutes moulding time, the foamed polyisocyanate/polyaddition product isdemoulded.

d) Precomminution

The demoulded article is precomminuted on a commercial cutting mill,obtainable e.g. from the Pallmann Co., Zweibrucken (FRG), Model PS-4-5,to give ca. 10 to 20 mm sized flakes, which are then introduced into theroller mill.

e) Pulverisation

In the following example, a laboratory roller mill of type SK 6612 fromBerstorff, Hannover (FRG), as shown in FIG. 1, was used. This machinehas two rollers with fixed bearings whose speeds can be independentlyregulated within the range 7 to 31.5 min⁻¹, with circumferences of 62 cmand lengths of 45 cm; the roll slit can be reduced to less than 0.1 mm.The rollers were not heated during the milling processes.

In the milling test the speeds of rotation were set at 30 min⁻¹ forroller 3 and 5 min⁻¹ for roller 2; roll slit 4 had a width of 0.1 mm. 1kg of the product--prepared as described above--was placed in themachine, a little at a time, and ca. 900 g of coarse flakes wereobtained directly under the pair of rollers 2, 3, which fall directlyfrom roll slit 4. These flakes still have a size of between 5 and 10 mm.Physically separated from this material, ca. 100 g of powder wereremoved from roller 3 using scraper 8. An average particle size of 200μm was determined for this powder.

f) Determination of particle size

The particle size was determined in a process using a light microscopewith subsequent transmission of the image to a computer followed byevaluation of the images using image evaluation software. Here, the areaof the particle at right angles to the direction of observationprojected onto the plane of observation is measured, this area beingapproximated to the area of a circle whose diameter is then calculated.This process gives good results, comparable to sieve analysis, for thoseparticles whose shape differs only slightly from that of spheres.

The average particle sizes quoted are the particle sizes (=diameter of asphere with the same mass) at which 50% of all particles are smaller and50% of all particles are larger than the value quoted.

We claim:
 1. A process for pulverizing polyurethane, polyurea and/orpolyurethane/polyurea materials in a roller mill in which at least tworollers rotate at different circumferential speeds comprising:a) passingprecomminuted material to be milled into a roll slit formed by therollers to form a coarse fraction and a separate sheet of fine fraction;b) collecting said coarse fraction falling freely downstream from saidroll slit; c) returning said coarse fraction to said roll slit; and d)scraping said sheet from one of said rollers so as to produce aplurality of discrete particles for further processing.
 2. A processaccording to claim 1, wherein the fine fraction is again classified intocoarse and fine particles and the coarse particles are returned to theroll slit.
 3. A process according to claim 1, wherein the sheet adheringto roller is passed directly to a roll slit formed between this rollerand a third roller by means of this roller.
 4. A process according toclaim 1, wherein the coarse fraction to be returned is cooled.
 5. Aprocess according to claim 1, wherein the material to be milled is apolyurethane rigid foam.
 6. A process according to claim 1, wherein thematerial to be milled is a polyurethane flexible foam.
 7. A processaccording to claim 1, wherein the material to be milled is apolyurethane structural foam.
 8. A process according to claim 1, whereinthe material to be milled is a RIM material made from polyurethane,polyurethane/polyurea or polyurea.